The invention relates to a device for influencing a laser beam by means of an adaptive mirror in the beam path.
Such a device is disclosed, for example, in DE 41 08 419 A1 for influencing the wave front and the cross-sectional geometry of the laser beam on the workpiece to be machined. Arranged for this purpose as near as possible to the laser beam source is a deflecting mirror with a mirror surface which ensures in conjunction with a minimum stroke of an actuator that the mirror is caused to bulge and produce a large beam expansion, in order thus to keep the focus spot diameter constant and/or to be able to adapt the machining parameters flexibly to the machining task. The actuator is constructed as a piezoelectric final controlling element which is arranged in a mirror housing. The piezoelectric final controlling element acts directly on the back of the mirror, it being possible as a result to achieve a rapid and exact alignment of the mirror.
However, such devices have the disadvantage that it is particularly difficult to achieve a pinpointed change or adjustment of the curvature of the mirror surface, as a result of which the optical quality of such systems is limited. This is based on the fact that the force for the curvature of the mirror is introduced at only one point on the back. It is therefore difficult to achieve a uniform flexure or surface curvature of the mirror, as a result of which the aberration of the mirror is increased. Moreover, such devices have the disadvantage that it is not possible for the mirror curvature to be adjusted in a damped or soft fashion. When the piezoelectric final controlling element is being driven, the change in length occurs suddenly, that is to say the force is likewise introduced suddenly onto the back of the mirror, as a result of which the quality of the optical system is limited.
A further principle of influencing a laser beam by means of an adaptive mirror is disclosed in DE 41 37 832 A1. In the case of this device or the adaptive mirror, the surface curvature is changed by employing the principle of a round plate to which a hydrostatic pressure is applied. Said pressure is applied in the centre of the mirror by a cooling liquid. The cooling liquid is removed via coolant discharge lines arranged at the edge of the mirror. The pressure of the cooling liquid is preferably controlled via electromagnetic valves.
Such systems have the disadvantage, however, that they have only a limited bandwidth for the rate of adjustment, which is limited to 50 Hz, for example. Such a system is excessively slow in reacting for a multiplicity of applications.
It is therefore the object of the invention to create a device for influencing a laser beam by means of an adaptive mirror in the beam path, in which a high optical quality is provided in conjunction with a high dynamic bandwidth of the rates of adjustment.
This object is achieved according to the invention by means of the characterizing features of Claim 1.
Because of the inventive combination of the two principles, that is to say the hydraulic deformation of the mirror and the piezoelectric deformation of the mirror, it is possible to create an optical system of high quality and, at the same time, a system having a high dynamic bandwidth of the frequency which can be modulated. Because of the deformation of the mirror by the hydrostatic pressure by means of a pressure-transmitting device, it is possible to achieve uniform introduction of force, and thus to achieve harmonic flexure of the mirror. Since the deformation pressure is applied via the pressure-transmitting device by means of piezoelectric final controlling element, there can be a high dynamic bandwidth for changing the mirror curvature, since the pressure-transmitting device acts indirectly or directly on the back of the mirror.
According to an advantageous embodiment of the invention, it is provided that the pressure-transmitting device has a largely incompressible, easily deformable intermediate body. This has the advantage that the exact and rapid change of the adjusting path of the piezoelectric final controlling element does not act directly on the back of the mirror, but directly or indirectly on the intermediate body, with the result that it is possible to preserve the soft hydraulic deformation and, above all, uniform introduction of forces for flexure.
According to a further advantageous embodiment of the invention, it is provided that the pressure-transmitting device has a pressure-transmitting member which acts on the intermediate body. Said pressure-transmitting member advantageously extends over the diameter or virtually over the diameter of the intermediate body. By contrast with the direct drive, via a piezoelectric final controlling element, it is possible as a result for the amplitude of the flexurized midpoint to be substantially higher than in the case of a direct drive, via the piezoelectric final controlling element. The reason for this is, in particular, that the volume of the intermediate body is substantially displaced by the pressure-transmitting member, preferably in the edge region. It is possible thereby on the basis of a smaller piezoelectric final controlling element with a smaller adjusting path to achieve a larger amplitude of the midpoint of the mirror with respect to the curvature, since a large quantity of liquid can be displaced owing to the large construction of the pressure member, as a result of which it is possible to enlarge the curvature at the midpoint of the mirror.
Further advantageous embodiments and developments are specified in the further claims.